A pipe mold is a long hollow cylindrical object with a bell and spigot end. Pipe molds are used in machines which centrifugally cast pipe. In centrifugal casting machines, the pipe mold is rotated on rollers at a high rate of speed while a stream of molten metal is introduced therein to form pipe.
Pipe manufactured by centrifugally casting are generally made from cast iron or ductile iron. These pipe are used, for example, in the ground for transporting water or natural gas. The pipe molds for making such pipe vary in size, having diameters ranging between 2 to 80 inches (50 to 2000 mm.) and lengths of up to about 30 feet (9 meters).
Pipe molds are often manufactured by forging. The larger pipe mold sizes are beyond the forging size capabilities of most forging suppliers. Where large forging equipment is available, the manufacturing costs are commercially prohibitive. In addition, there is a need to heat-treat the forged pipe mold in a furnace to achieve the desired mechanical and physical properties, such as hardness. In some grades of steel, however, only a low depth of hardening is possible by heat-treatment.
The size limitations of forging can be alleviated by manufacturing pipe molds by forming a flat rolled plate into a cylindrical form and welding the resulting seam or joint in the longitudinal or helical direction. When long pipe molds are desired, several tubes can be butt-welded together circumferentially at their ends to make a pipe mold of a desired length.
One disadvantage in using a welded form, however, is that each welded joint creates an area of weakness in the mold called a "metallurgical notch." The weakness of the metallurgical notch is due to (1) grain mismatch between the plate and the weld and (2) grain coarsening in the "heat affected zone" near the weld which occurs due to the heat of welding. In addition, hoop stresses caused by the constantly changing hot and cold temperatures during the casting process tend to cause the metallurgical notches to open up.
Therefore, during service, the pipe mold is prone to fail or crack along the metallurgical notches. The molten iron used in making the pipe may seep into the cracks in the pipe mold. When the cast pipe is pulled out of the pipe mold, it will further damage the internal surfaces of the pipe mold. Not only does this cracking cause the pipe mold to wear out, but also a pipe made with a cracked mold will have defects on its outer wall.
The pipe mold failures described above result in considerable expense to pipe manufacturers because such pipe molds must be frequently repaired or replaced.